lost power

[MX3KEV21]

my k8 mx3 has lost power it just does'nt seem to pull that quickly past 80 or 90mph and the engine when finally getting it to 110mph it wont pull anymore at all. also the engine bucks when stepping on the gas when above 50mph but not below. also when idling in neutral or gear if i tap the accelerator the revs dip then rise. i have traced the engine pink/ping to possible valve stem seals leaking. but would this be enough to cause all the other problems? i have replaced all the usual things but not the two o2 sensors. thanks in advance

[Custom_V6_Limited_SE]

Have you checked your ignition timing?

[MX3KEV21]

yeah the ignition timing is spot on as is the cam timing. fuel filter, leads spark plugs, dizzy cap, rotor arm, both coolant sensors, tps setup new cat and back box and all vacuum lines checked and ok. the only thing that has'nt been checked is the fuel pressure and the compression which are getting done next week. the car sat for a year with a couple of litres of fuel in the tank and not being started.